A flow measuring device outputs from a flow sensor an electrical signal in accordance with a flow rate. The outputted electrical signal is then converted into the flow rate through a predetermined conversion map. For example, an electronic control unit (ECU) connected to the flow measuring device includes a storage means for storing the conversion map, and a calculation means for converting the electrical signal into the flow rate based on the conversion map.
A variation exists for each product in output characteristics from the flow sensor. For this reason, for example, before shipment of the product or before attachment of the sensor to an air intake duct, the output characteristic needs to be adjusted to a desired output characteristic that is necessary to convert through a predetermined conversion map.
Conventionally, a technology of adjusting the variation for each product in the output characteristics with respect to a forward flow (flow from the upstream side toward downstream side in a mainstream) is implemented. However, a variation for each product in the output characteristics with respect to a backward flow has not been considered. Accordingly, as illustrated in FIG. 10, the output characteristic varies between products in a backflow region, and these output characteristics are converted through a common conversion map, which results in low accuracy.
Technologies which can detect a flow rate of the backflow region and corrects a conversion map when converting an output in the backflow region are described in Japanese Patent No. 3583136 (corresponding to U.S. Pat. No. 5,668,313) and JP-A-H09-015013 (corresponding to U.S. Pat. No. 5,681,989). Nevertheless, there is no description as to adjustment of the variation for each product in the output characteristics in the backflow region.